Bone is the principal storage tissue for lead in the human body. More than 95% of the body lead burden is contained in bone, associated closely with calcium. During pregnancy, bone calcium sires are extensively mobilized. We hypothesize that concurrent with this calcium mobilization there occurs substantial mobilization of bone lead. Because lead can easily traverse the placenta, we hypothesize further that this lead mobilized from bone serves as a substantial internal source of toxic exposure for the developing fetus. To test these hypotheses, 1,200 non-pregnant, nulliparous women aged 20-29 years who have applied for a marriage license between October 1993 and December 1994 at the Marriage Registration Bureau in 5 Polish cities (total population 600,000) with known high environmental sources of lead will undergo blood lead screening. This screening will occur in the context of an ongoing collaboration between the Mount Sinai School of Medicine and the Silesian Medical Academy, Katowice, Poland. From this screened population, 180 women with baseline blood lead levels > 1O microgram/dl and 180 with blood lead levels of equal to or less than l0 micrograms/dl will be invited to undergo measurement of bone lead content by X-ray fluorescence (XRF) spectroscopy. All women selected for XRF analysis will be required to have had a negative urine pregnancy test within the preceding week. Members of this cohort of 360 women who become pregnant during the 15 month enrollment period (an estimated 50%) will have blood lead measurements performed at the end of their first and second trimesters and again in the immediate post-partum period. Cord blood lead determinations will be performed on newborn infants. During the 10-day period following delivery these women will have a second bone lead measurement. Final blood and bone lead measurements will be carried out 9 to 21 months post-partum. To assess mobilization of lead from bone, the difference in bone lead level before and after pregnancy will be examined, and a toxicokinetic estimate will be developed of the total amount of skeletal lead released. The relation between maternal bone and blood lead levels will be examined. A correlation will be sought between the amount of lead mobilized from bone and the cord blood lead level of the infant. If our study demonstrates that appreciable amounts of bone lead are mobilized during pregnancy, we plan to pursue additional studies to determine whether this lead adversely affects in utero neurodevelopment.